1. Field of the Invention
The present invention describes a process for preparing (meth)acrylic esters by acid-catalyzed esterification of (meth)acrylic acid or by transesterification of (meth)acrylic esters.
2. Description of the Background
In this document, the term (meth)acrylic acid is an abbreviation of methacrylic acid and/or acrylic acid, and (meth)acrylic ester of methacrylic ester and/or acrylic ester.
The polymers and copolymers prepared based on (meth)acrylic esters are of great economic importance in the form of polymer dispersions. They find use, for example, as adhesives, paints or textile, leather and paper assistants.
(Meth)acrylic acid and (meth)acrylic ester are polymerizable compounds. Care therefore has to be taken in all process steps that there is sufficient polymerization inhibition. As a consequence of the large amounts of heat released, undesired polymerization is dangerous to safety. Examples of such runaway reactions are described, for example, in Process Safety Progress 1993, Vol. 12, 111-114 and Plant/Operations Progress 1987, Vol. 6, 203-207.
The preparation of (meth)acrylic esters in laboratory experiments is only reproducible to a limited extent. Some reactor charges polymerize unexpectedly. This effect occurs in particular when relatively old alcohols are used for the esterification or transesterification. Relatively old alcohols refer to alcohols which are not used directly after their preparation and are stored for an indefinite time, generally several days. These may contain peroxidic impurities which lead to the polymerization of (meth)acrylic acid or (meth)acrylic ester.
JP 2001/011018 describes the thermal treatment of cyclohexanol in the presence of water to remove peroxidic impurities. Subsequently, a cyclohexanol treated in this way is used for esterification or transesterification to cyclohexyl (meth)acrylates. A disadvantage of this process is that the alcohol contains water after the treatment. This has to be additionally removed in the course of the acid-catalyzed esterification. In the course of the transesterification, the catalysts used (for example titanium alkoxides) are destroyed by water.
In addition to the alcohols, (meth)acrylic acid is also capable of forming peroxides with atmospheric oxygen. This is especially significant because (meth)acrylic acid is blanketed with air in the course of storage for reasons of sufficient polymerization inhibition (Plan/Operations Progress 1987, Vol. 6, 188-189). For example, the peroxide number in acrylic acid stored for six months after HCl digestion is approx. 0.8 meq/kg (organic peroxide).
The situation is similar for (meth)acrylic ester also. As described by Levy in Journal of Applied Polymer Science 1996, Vol. 60, 2481-2487, organic peroxides are formed in the course of storage of butyl acrylate.